A desmodromic valve train has traditional cams opening the valves, and cmas and fingers closing the valves. This form of technology is very meture, and if properly assembled and the tolerances are proper, very reliable and give great power. The traditional valve spring is eliminated, and replaced by a much lighter spring used just to seat the valve in the last few millimeters (or less). As mentioned by joseff, there is a lot less spring to fight, and frictional losses are much lower, especially at lower RPM. Usually, because a finger is required to close the valve, we rarely see the camshaft riding directly over the valve, but instead beside it, with rockers or fingers on pivots.
Pneumatic valves eliminatwe the traditional valve spring, allowing much less mass in the valvetrain, thus higher RPM's. They can be very compact. But they do occasionally develop leaks, and during a race that requires recharging the pressurized bottle in pit stops. And before you start an engine, it is critical you have pressure, or all the valves will be instantly destroyed when the pistons rise. That is why they have a severe handicap in road car engines.
They are very different systems, each with strengths and handicaps.